Fort de Bertheaume, Coastal fortress in Plougonvelin, France.

Fort de Bertheaume is a fortress on a rocky islet linked to the shore by a footbridge, positioned at the entrance to the Iroise Sea. The structure features multiple levels stacked upon each other showing different building techniques and remains accessible through a small bridge crossing.

A castle stood here by the 15th century, first recorded in an act of 8 March 1474; an Anglo-Dutch force of 7,000 men destroyed it in July 1558. Vauban set up a battery in 1689 to bar the Goulet de Brest, and it fired on the British during the Camaret attack of 1694. Construction of a larger fort began in 1700 under Benoît de Musinot, with a bastion, a semi-circular battery, a magazine, barracks and a powder store.

The name Bertheaume probably derives from the Breton "aber C'homm", the bay of the valley, though another tradition links it to Saint Bertem, invoked during the English custody of the castle in the 14th century. On the mainland part of the site, a grassy theatre ringed by rows of small standing stones hosts sound-and-light shows retracing the fort's history, concerts, and the Visions festival on the first weekend of August.

The islet is reached by a footbridge first built in 1835; before that a nacelle was the only link, and winter storms have repeatedly damaged or carried away the walkway, so it has been rebuilt several times. The site is shared with a via ferrata and zip lines run by Bertheaume Iroise Aventure, whose course takes about 1 h 30 min. The mainland part holds an open-air theatre ringed by rows of small menhirs set up in the 1990s.

The fort's name may come from Saint Bertem, or Bertellin, called Berteaume, during the English custody of Brest in the 1360s; the Breton name is Kastel Persel. The site also gave its name to the Berteaume Mesolithic, a flint period defined by tools found on the cliff.